1. Field of the Invention
This invention relates to a method and apparatus for affixing a prosthesis to bone and, in particular, to a method and apparatus for affixing a prosthesis to bone so as to produce stress transfers between the prosthesis and the bone which generally correspond to the stress transfers which occur in natural bone.
2. Description of the Prior Art
The bony portion of human and animal bones consists of two types of tissue: hard or compact bone which is dense in texture, and soft or cancellous bone which consists of fibers and lamellae joined together to form a reticular network. The hard bone makes up the outer wall of the bone and provides most of the bone's overall strength. Cross-sections at all levels through a bone will include some hard bone. The thickness of the hard bone will vary with the level of the cross-section, being smallest near the ends of the bone and greatest at the middle.
Soft bone, where present, forms the inner core of the bone. This type of bone is primarily found near the ends of bone where the hard bone is thinnest. Current understanding is that, in these regions, the soft bone contributes to the overall strength of the bone by transferring at least some of the applied stresses from the thin portions of the hard bone to relatively large areas of the thicker portions of the hard bone located closer to the middle of the bone. In these regions of stress transfer, the fibers making up soft bone appear to have a regular, equipotential-like, arrangement wherein fibers intersect the hard bone at spaced intervals of approximately 1-2 mm. It is believed that this arrangement, at least in part, is responsible for the efficient transfer of applied stress from one part of hard bone to another.
To date, the methods and apparatus used to attach prostheses to bone have not adequately taken into account the detailed anatomy of bone described above. Specifically, the prior art approaches have failed to provide stress transfer to large areas of the thicker portions of hard bone as occurs in nature through the interaction of soft bone with hard bone. Similarly, the fact that soft bone transfers stress to hard bone at spaced intervals of approximately 1-2 mm has also been ignored.